JP3201513B2 - Hot rolled steel sheet cooling method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method and an apparatus for cooling a hot-rolled steel sheet, and more particularly to an efficient method for cooling a thick steel sheet.

[0002]

2. Description of the Related Art In recent years, the on-line controlled cooling method has been increasingly applied as a process for manufacturing thick steel plates. This is a method of cooling the hot steel sheet immediately after rolling online. By using this method, in addition to the effect of imparting high strength and high toughness, the cost reduction effect such as the reduction of alloy elements and heat treatment is reduced. can get.

[0003] However, in general, hot rolled steel sheets after rolling are not necessarily uniform in temperature distribution, sheet shape, surface properties, and the like, so that uneven cooling is likely to occur in the steel sheet surface during cooling.
Deformation, residual stress, material unevenness, etc. occur in the steel sheet after cooling,
This leads to poor quality and operational problems.

In general, when a high-temperature steel sheet is cooled, first, the steel sheet enters a film boiling state in which a vapor film exists between the surface of the steel sheet and cooling water. This is a boiling phenomenon in which the cooling water evaporates before reaching the steel sheet surface due to the extremely high surface temperature of the steel sheet and does not come into direct contact.There is always a vapor film between the steel sheet and the cooling water. Low heat flux and low cooling capacity.

[0005] Next, when the surface temperature of the steel sheet falls, a transition from film boiling to transition boiling occurs. In this transition boiling region, the stable steam film covering the steel sheet surface can no longer exist stably, and the steam film collapses locally, causing direct contact between the cooling water and the steel sheet surface, and the heat flux rapidly increases. Increase. At this time, if the surface in which the steam film collapses and the cooling water comes into direct contact with the steel plate is not uniform, temperature unevenness may occur. That is, a portion where the steam film is locally broken and the cooling water directly contacts the steel plate is a low temperature portion, whereas a portion where the steam film exists is a relatively high temperature portion.

[0006] Once the cooling unevenness occurs on the surface of the steel sheet, in the transition boiling region, the cooling is promoted because the heat flux is large in the low temperature part, whereas the high temperature part is in the low temperature part. The cooling is delayed because the heat flux is small. As a result, the temperature difference between the two increases. The temperature unevenness that occurs during cooling occurs almost in this region.

[0007] When the temperature is further lowered, a steam film cannot be present any more, and almost the entire surface of the steel sheet comes into contact with the cooling water, ie, nucleate boiling. In this region, since almost the entire surface is in contact with the cooling water, temperature unevenness such as occurs in the transition boiling region hardly occurs.

In order to suppress the occurrence of such uneven cooling of the steel sheet, a number of uniform cooling methods have been proposed. Among them, the cooling method in which cooling water flows from the slit-shaped nozzle in the sheet width direction in the traveling direction of the steel sheet, in addition to the high cooling ability, a uniform cooling ability distribution in the width direction is obtained. It is also applied to online cooling equipment. This is achieved by discharging a uniform cooling water flow in the width direction from the slit-shaped nozzle at high pressure and at a large flow rate.
This is because a steam film is not generated on the entire surface of the steel sheet, an unstable transition boiling region can be avoided, and the steel sheet can be promptly shifted to the nucleate boiling state.

[0009]

However, this requires a large amount of cooling water, and if this system is applied to all cooling zones, a large investment is required for a cooling water supply system and a drainage system. On the other hand, if the amount of water is reduced, the steam film cannot be broken uniformly over the entire surface of the steel sheet, and a portion where the steam film remains locally occurs, which causes uneven temperature of the steel sheet. Therefore, it is an excellent feature of this method.
In order to obtain uniform cooling capacity in the width and longitudinal directions,
In fact, high-pressure, large-flow cooling water was required, and other than that, stable and even cooling was not possible.

Japanese Unexamined Patent Publication (Kokai) No. 61-264137 discloses that in a slit jet cooling system, the amount of water is 150 to 20.
A method is disclosed in which 0 m ³ / hm, the discharge pressure is 1.5 to ² kg / cm ² , and the angle of the slit nozzle is 15 to 25 °.

FIG. 6 shows this method. In FIG. 6, 1
, A table roll (lower roll), 3 a draining roll (upper roll), 4 a slit nozzle, 5 a slit nozzle header, 6 a spray nozzle, and 7 a spray nozzle header.

As shown in FIG. 6, the cooling device has two stages and is divided by a drain roll 3. The cooling is performed by the primary cooling device using a slit jet method (cooling method using the slit nozzle 4) and the secondary cooling device using another cooling method (cooling method using the spray nozzle 6). On this occasion,
In the primary cooling device, a large amount of cooling water is injected from the slit nozzle 4 toward the traveling direction of the steel sheet 1 along the upper surface of the steel sheet 1. The present invention limits the amount of water, water pressure, and the like required to uniformly remove a vapor film by a cooling method using a slit jet method and shift to stable cooling. However, when the secondary cooling device is cooled by another cooling method, there is a problem that the surface temperature of the steel sheet is recovered, the transition boiling state becomes unstable again, and cooling unevenness occurs. In particular, in the case of a thick steel plate, this problem is remarkable because the amount of recuperation is large.

Further, if the slit jet method of the primary cooling device is applied to the secondary cooling device as it is, the occurrence of uneven cooling can be suppressed, but a large amount of cooling water is required, and the supply of cooling water is required. A large capital investment was required for the system and drainage system, and the economic disadvantage was unavoidable. Furthermore, when the cooling block is partitioned by the draining device, no guide is given regarding the amount of water and the pitch of the draining device.

The present invention has been made in order to solve the above-mentioned problems. In a cooling system in which cooling water flows from a slit-shaped nozzle in the traveling direction of a steel sheet, the cooling system has An object of the present invention is to reduce the amount of cooling water and perform efficient cooling while maintaining a uniform cooling capacity.

[0015]

The object of the present invention is to dispose a plurality of cooling blocks divided by draining blocks in the direction in which the steel sheet passes, and in each cooling block, install a slit-shaped nozzle above the steel sheet. In the method for cooling a hot-rolled steel sheet, in which cooling water flows from the slit-shaped nozzle through the slit-shaped nozzle, the flow velocity Vw (m / s) of the cooling water flowing out of the slit-shaped nozzle is reduced by draining from the slit-shaped nozzle. The distance to the device is L (m) and the passing speed of the steel sheet is Vs
(M / s), where α (m / s) is a correction term based on the steel sheet surface temperature, the problem is solved by a method for cooling a hot-rolled steel sheet, which is in a range represented by the following equation.

10 ≧ Vw ≧ kL + Vs + α where k = 1.0 (s ⁻¹ ) where α is the surface temperature of the steel sheet as Ts (° C.).
It is represented by the following equation. α = aTs ² + bTs + c where a = 5.0 × 10 ⁻⁶ (ms · ^{1 −1} ° C. ⁻² ) b = −0.0013 (ms · s ^-1 ° C. ⁻¹ ) c = 0.29 (m · s s- ¹ )

(Circumstances leading to the invention) As a result of studying the above objects, the present inventors have found that each cooling block is partitioned by a drainer, and then cooling water is caused to flow in the traveling direction of the steel sheet by slit-shaped nozzles. In cooling, the conditions necessary to suppress the generation of a vapor film and to uniformly transition to the nucleate boiling state over the entire cooling surface are not only the amount of cooling water but also the flow rate of the cooling water and the actual flow from the nozzle to the draining device The present invention has been found to depend on the distance of the steel sheet and the surface temperature of the steel sheet.

FIG. 1 is a view for explaining the state of cooling water on the surface of a steel plate at the initial stage of cooling when cooling is performed by flowing cooling water from a slit-shaped nozzle in the traveling direction of a high-temperature steel plate using a draining roll as a water draining device. is there.

In FIG. 1, 1 is a steel plate, 2 is a table roll (lower roll), 3 is a draining roll (upper roll), 4
Represents a slit nozzle, 5 represents a slit nozzle header, 9 represents a nucleate boiling region, 10 represents a film boiling region, X represents a transport direction of the steel sheet 1, and Y represents a direction in which cooling water flows. L is the distance L from the slit-shaped nozzle to the draining device in the present invention.
(M) ".

In the vicinity where the cooling water discharged from the slit nozzle 4 collides with the steel sheet, the vapor film disappears instantaneously and easily becomes a nucleate boiling state, but as the cooling water flows along the steel sheet surface, the water temperature becomes higher. Rises, and when the supply of new cooling water is insufficient, that is, when the flow rate is low, a vapor film is generated near the middle of the nozzle and the draining roll, and the film moves to the film boiling region 10. The region where the vapor film is generated is not constant, but is generated locally locally and unstablely due to the surface condition and the temperature unevenness of the steel sheet, which causes uneven cooling.
Then, since the cooling water collides with the draining roll, the cooling water is in a turbulent state near the draining roll, the generation of a vapor film is suppressed, and the nucleate boiling state is established again.

When the present inventors examined the time required for the vapor film on the cooling surface of the hot steel sheet to disappear when cooling water was cooled by flowing cooling water from the slit-shaped nozzle in the traveling direction of the hot steel sheet, It was found that the time required for the vapor film to disappear depends on the flow velocity of the water and the drain roll pitch.

FIG. 2 is a graph showing the relationship between the flow rate of cooling water and the time until the vapor film on the cooling surface disappears in a steel plate having a thickness of 30 mm and a temperature of 900 ° C. From this figure, it can be seen that as the distance from the slit nozzle 4 to the draining roll 3 is shorter, the vapor film can instantaneously disappear at a lower flow rate.

Further, when this tendency was examined by changing the surface temperature of the steel sheet, it was found that the flow rate at which the vapor film on the cooling surface instantaneously disappeared also changed depending on the surface temperature of the steel sheet. FIG. 3 shows the minimum flow velocity V required for the vapor film to disappear instantaneously when a steel plate having a thickness of 30 mm is cooled while changing the temperature.
wm (m / s) and distance L (m) from nozzle to draining roll
FIG. From this figure, it can be seen that the flow rate of the cooling water required to instantaneously eliminate the vapor film has a linear relationship with the distance from the nozzle to the draining roll even if the steel sheet surface temperature changes. Also, it can be seen that as the steel sheet surface temperature is lower, the vapor film can be instantaneously eliminated at a lower flow rate.

From the above, Vwm (m / s) is the minimum flow velocity required for instantaneous disappearance of the vapor film, L (m) is the distance from the nozzle to the draining roll, and α is a correction term based on the steel sheet surface temperature. (M / s), Vwm = kL + α where k = 1.0 (s ⁻¹ ).

The relationship between the surface temperature of the steel sheet and the flow rate of the cooling water required to instantaneously eliminate the vapor film is obtained from FIG. 3, and α is as follows when the surface temperature of the steel sheet is Ts (° C.). It can be understood that it can be represented.

Α = aTs ² + bTs + c where a = 5.0 × 10 ⁻⁶ (ms · ^{1 −1} ° C. ⁻² ) b = −0.0013 (ms · s ^-1 ° C. ⁻¹ ) c = 0.29 (M · s ^-1 ) In addition, when flowing cooling water in the direction of travel of the steel sheet, it is necessary to consider the passing speed of the steel sheet. The range of the flow rate of the cooling water can be expressed as Vw ≧ kL + Vs + α, where Vs (m / s) is the sheet passing speed.

Here, as shown in FIG. 4, the surface temperature of the steel sheet that has entered the cooling zone repeats its amplitude and decreases each time it passes through each cooling block. This is because the cooling capacity changes at the portion of the table roll or draining roll between the cooling blocks. This steel sheet surface temperature profile is calculated in advance based on the cooling capacity of the cooling surface,
It is possible to predict. Therefore, for each cooling block, if the surface temperature of the entering steel sheet is calculated in advance and predicted, it is possible to set the minimum flow velocity necessary for instantaneously shifting to the nucleate boiling state at that surface temperature. It becomes possible.

In order to control the cooling rate, it is possible to set an arbitrary cooling rate by stopping the cooling of a plurality of cooling blocks and performing intermittent cooling on and off. . In this case, too, by previously calculating the steel sheet surface temperature in each cooling block during intermittent cooling in advance, the cooling surface can be instantaneously shifted to the nucleate boiling state with the required minimum amount of water to suppress cooling unevenness. Meanwhile, the cooling rate can be controlled. The cooling stop temperature can be controlled by the transport speed.

Further, the flow velocity Vw of the cooling water is set to 10 m / s or less. This is because, when the flow velocity exceeds 10 m / s, the cooling water colliding with the steel plate rebounds, causing uneven cooling, and the amount of cooling water becomes enormous, which is economically disadvantageous.

[0030]

EXAMPLE The present invention was implemented using the apparatus shown in FIG.
5, the same elements as those shown in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted. 8 is a flow control valve. In FIG. 5, there are a total of 20 cooling blocks, and each cooling block is separated by a drain roll 3 so that cooling water does not enter an adjacent cooling block. A slit nozzle 4 is provided on the upper surface of the steel plate of each cooling block.
Is installed, and cooling water is discharged from the nozzle along the traveling direction of the steel sheet. The diameter of the draining roll 3 is 250 mm, the interval is 900 mm, and the distance from the tip of the slit nozzle to the draining roll is 600 mm.

A plurality of spray nozzles 6 are arranged on the lower surface of the steel plate. The cooling method for the lower surface of the steel sheet is not limited to spray cooling, but may be a slit jet method, a perforated nozzle method, a circular pipe fountain method with a conduit, an immersion cooling method, or a combination thereof. However, as in the case of the upper surface, it is necessary to control the water density so as to maintain the nucleate boiling state.

The collision position and the cooling area of the cooling water discharged from the upper and lower nozzles are set to be the same on the upper and lower surfaces of the steel plate. This is to make the temperature histories of the upper and lower surfaces of the steel sheet coincide with each other and to suppress deformation of the steel sheet during cooling.

The angle between the slit nozzle 4 and the steel plate 1 is 14 °
The distance between the slit nozzle 4 and the steel plate is 30 mm, the gap between the nozzles is 5 mm, and the discharge pressure of the cooling water is 0.1 to 1.0.
kg / cm ² .

The steel sheet 1 rolled by the rolling mill is conveyed by a table roll 2 and conveyed to a control cooling device. At this time, although not shown in the drawing, rolling distortion may be removed in advance by a hot leveler before entering the controlled cooling device. The steel sheet transported to the control cooling device is injected with cooling water from above and below while passing through the cooling zone, and is cooled to a predetermined temperature. The intermittent pattern of the cooling block supplying the cooling water and the intermittent pattern of the cooling block not supplying the cooling water was calculated in advance, and the cooling rate was controlled to a desired value by the intermittent pattern. The amount of cooling water is calculated in advance according to the intermittent pattern by calculating the surface temperature of the steel sheet when entering each cooling block, and according to the surface temperature of the steel sheet, the minimum amount of cooling water required to instantaneously shift to the nucleate boiling state Was set based on the formula of the present invention.

Using the example of the present invention, a plate thickness of 40 mm and a plate width of 30
Table 1 shows the amount of strain and the amount of water used after cooling a steel sheet having a length of 2000 mm and a length of 20000 mm under various conditions.

[0036]

[Table 1] (Table 1)

The strain of the steel sheet after cooling was represented by the ratio of the deformation of the steel sheet in the thickness direction to the sheet length. The lower the value, the smaller the amount of distortion and the better the flatness. Further, as a comparative example, a result when cooling is performed with a constant flow rate is also shown.

From Table 1, it can be seen that in the present invention, the minimum amount of water required to instantaneously shift the surface of the steel sheet to the nucleate boiling state can be set in each cooling block. Can be reduced.

In the embodiment, No. 1 uses only # 1, 4, 7, and 13 cooling blocks by intermittent cooling, and based on the steel sheet surface temperature predicted by calculation, the respective flow rates are 6 m / m. s, 2.5m / s, 2.5m / s, 1.5m / s, 1.5m /
s. In this case, in each cooling block, the minimum amount of water required to instantaneously shift the steel sheet surface to the nucleate boiling state can be set, so that the cooling distortion was small and the amount of water used was also small.

On the other hand, in Comparative Example No. 4, the same cooling block was used. However, since the flow velocity in each cooling block was uniformly set at 2 m / s, the total amount of cooling water was slightly reduced. Unevenness occurred and cooling strain increased. This is because when the steel sheet was in a high temperature state, it was not possible to make the nucleate boiling state uniformly over the entire surface in the cooling block.

In the embodiments, No. 2 and No. 3 use all the cooling blocks and control the cooling stop temperature by changing the transport speed. At this time, as in the case of No. 1, the flow velocity of each cooling block was set based on the steel sheet surface temperature predicted by calculation. Specifically, 1.6 m / s in the # 1 cooling block, and 3.8 m / s in the # 2 cooling block.
m / s, 2.5 m / s, # 3 to # 8 cooling block
For the cooling blocks 9 to # 13, 1.0 m / s, # 14 to #
In a 20 cooling block, it was set to 0.5 m / s.

Also in this case, in each cooling block, the minimum amount of water required to instantaneously shift the steel sheet surface to the nucleate boiling state can be set, so that the cooling distortion can be reduced and the amount of water used can be reduced at the same time. Was.

[0043]

In Comparative Example No. 4, the flow velocity in all cooling blocks was 4 m / s. In this case, since the steel sheet cannot be brought into the nucleate boiling state uniformly over the entire surface in the cooling block at a high temperature, cooling distortion occurs,
Cooling distortion increases, and the amount of water used increases, which is economically disadvantageous.

[0045]

As described above, according to the present invention,
In the cooling method and apparatus in which the cooling water is caused to flow in the traveling direction of the steel sheet by the slit-shaped nozzle, the minimum amount of the required cooling water is obtained, and the cooling of the steel sheet is performed with the cooling water amount close to the required amount. While maintaining a uniform cooling capacity in the direction, it is possible to greatly reduce the amount of cooling water used. Therefore, it is not necessary to make a large capital investment for the supply system and the drainage system of the cooling water, and there is a great economic effect. This is extremely effective in reducing the distortion of the steel sheet after controlled cooling,
An economical cooling method and apparatus can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the state of cooling water on the surface of a steel sheet.

FIG. 2 is a diagram shown.

FIG. 3 is a diagram showing a relationship between distances to a roll.

FIG. 4

FIG. 5

FIG. 6

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2 Table roll 3 Drain roll 4 Slit nozzle 5 Slit nozzle header 6 Spray nozzle 7 Spray nozzle header 8 Flow control valve 9 Nucleate boiling area 10 Film boiling area

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shogo Tomita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-63-168215 (JP, A) JP-A-9 -19711 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B21B 45/02 320 C21D 9/52 102

Claims (1)

(57) [Claims] 1. A plurality of cooling blocks divided by draining blocks are arranged in the direction in which the steel sheet passes, and in each cooling block, a slit-shaped nozzle is installed above the steel sheet. In the method for cooling a hot-rolled steel sheet in which cooling is performed by flowing along a flow path, a flow velocity Vw (m / s) of cooling water flowing out of the slit-shaped nozzle is provided.
Is the distance from the slit nozzle to the draining device
(M), when the sheet passing speed of the steel sheet is Vs (m / s), and the correction term based on the steel sheet surface temperature is α (m / s), the heat is in a range represented by the following equation. Cooling method for hot rolled steel sheet. 10 ≧ Vw ≧ kL + Vs + α where k = 1.0 (s ⁻¹ ) where α is the surface temperature of the steel sheet as Ts (° C.).
It is represented by the following equation. α = aTs ² + bTs + c where a = 5.0 × 10 ⁻⁶ (ms · ^{1 −1} ° C. ⁻² ) b = −0.0013 (ms · s ^-1 ° C. ⁻¹ ) c = 0.29 (m · s s- ¹ )